The invention is in the field of electrical switches, in particular, electrical switches relying on a liquid metal or metal alloy as an electrical conducting material for bridging an electrical gap between electrodes.
Electrical switches and sensors that are usually referred to as xe2x80x9ctiltxe2x80x9d switches, such as thermostats, float controls, solenoids, relays, etc. are commonly used in a variety of electrical applications. The making or breaking of electrical contact in these switches, hence the electrical switching action, is generally accomplished by mechanical movement of the switch which causes a quantity of a bridging conducting material contained therein to flow from one location to another. Liquid mercury is used extensively in such electrical switches and sensors as the bridging conducting material.
In a typical switch application, a quantity of liquid mercury, or xe2x80x9cmercury melt poolxe2x80x9d is positioned inside a capsule or housing into which spaced apart electrodes or electrical contacts extend. Depending on the physical orientation of the housing, the mercury melt pool can provide a conductive pathway between the electrodes, or be positioned such that there is an open circuit between the electrodes. The switch is closed and electrical contact made when the switch housing is moved in a manner such that the mercury melt pool flows toward a location in the housing at which the mercury bridges the spaced electrodes, thereby permitting the flow of electricity from one electrode to the other. Conversely, the switch is opened and electrical contact broken when the switch housing is moved in a manner such that the mercury melt pool flows towards and collects at a different position in the switch housing out of contact with at least one of the electrodes.
An important physical attribute of mercury for the purposes of electrical switch applications, aside from its ability to conduct electricity, is that it remains fluid throughout a wide temperature range thus enabling it to be used in many different environments, or in environments with constantly changing temperature parameters. Another important physical attribute of mercury is that it has significant surface tension and does not wet many surfaces that it contacts, such as glass, metal or polymer surfaces, it shows a high sensitivity to tilting of the switch housing, and it generally does not become damaged by contact with the electrodes.
A problem with mercury-based electrical switches is that mercury is toxic to humans and animals, and exposure to mercury is a significant concern in any application or process in which it is used. Utilization of mercury during manufacturing may present a health hazard to plant personnel, and the disposal of devices that contain mercury switches or the accidental breakage of mercury switches during use may present indirect hazard to people within the immediate vicinity of the switch.
As a result of the toxicity of mercury, non-toxic replacements for mercury in electrical switch applications have been sought. A candidate for replacing mercury in electrical switches is liquid gallium metal or liquid gallium alloys. For example, U.S. Pat. No. 3,462,573 (Rabinowitz et al.) and Japanese Patent Application Sho 57-233016 to Inage et al. each disclose that gallium or gallium alloys may be useful as a replacement for mercury in electrical switches.
However, while gallium is non-toxic, it does not have all of the beneficial properties of mercury. For example, gallium and gallium alloys tend to have a significantly lower density and a lower surface tension than mercury, which may result in inferior contact angles between the gallium or gallium alloy melt pool and the wall of the switch capsule, which is commonly prepared glass. The contact between the gallium or gallium alloy melt pool and the glass of the capsule is generally broader than the contact between a mercury melt pool and glass, resulting in a greater amount of drag acting on the gallium or gallium alloy melt pool. This, coupled with the lower density of the gallium or gallium alloy, means that a switch having gallium or gallium alloy as the bridging conducting material may not be as sensitive to tilt as a comparable mercury based switch.
In view of the drawbacks and problems with the prior art, a need exists for a novel approach to gallium-based electrical switch construction which will reduce or eliminate such drawbacks and problems.
In some embodiments of the present invention, there is provided an apparatus for making and breaking an electrical connection in an electrical circuit, the apparatus comprising a housing of an electrically non-conducting material, the housing defining a sealed cavity, at least two spaced electrodes, each electrode extending through the housing into the cavity, a contact member within the cavity, the contact member comprised of a core member having a layer of liquid gallium or liquid gallium alloy surrounding the core member wherein the layer of gallium or gallium alloy adheres to the core member, the contact member being moveable within the cavity of the housing in response to movement of the housing to electrically connect any two electrodes by positioning the gallium or gallium alloy layer in contact the two electrodes, and to electrically disconnect the two electrodes by positioning the gallium or gallium alloy layer out of contact with any one of the electrodes.
In some embodiments, the core member may be copper or a copper-coated material. In some embodiments, the gallium alloy may be an alloy of gallium, indium and tin, for example and alloy comprised of gallium in the range of approximately 60-75% of the alloy by weight, indium in the range of approximately 15-30% by weight, and tin in the range of approximately 1-16% by weight. In further example, the gallium alloy may be a eutectic alloy of 62.5% gallium, 21.5% indium, and 16% tin.
In some embodiments, the switch housing may be under vacuum, or may contain an inert atmosphere, such as a noble gas to prevent the oxidation of the gallium.
In some embodiments, the switch housing may contain a fluid containing NH3, for example an aqueous solution of NH4OH or gaseous mixture.
In some embodiments, there is provided an electrical switch comprising a housing of an electrically non-conducting material, the housing defining a sealed cavity, at least two spaced electrodes, each electrode extending through the housing into the cavity, a contact member within the cavity, the contact member comprising a core member of density greater than 6 g/cm3 and a layer of liquid gallium alloy surrounding and wetted to the core member to pull the gallium alloy meniscus away from the housing, the contact member being moveable within the cavity of the housing in response to gravity to electrically connect any two of said at least two electrodes by positioning the gallium alloy layer in contact with said any two of said at least two electrodes, and to electrically disconnect said any two of said at least two electrodes by positioning the gallium alloy layer out of contact with any one of the two electrodes. In some embodiments, the core member may have a density greater than 9 g/cm3.